Various lines of evidence have implicated the importance of the interstitial fluid compartment in arterial pressure control in human hypertension. Also recent studies have indicated that alterations in interstitial fluid compartment compliance might play a direct role in the pathogenesis of experimental hypertension. The goals of this study are to examine in three different forms of experimental hypertension: 1) whether interstitial fluid compartment compliance is different from that in the appropriate normotensive control animals; 2) whether the progression of hypertension is associated with progressive changes in interstitial fluid compartment compliance; and 3) whether the changes in interstitial fluid compartment compliance after chronic volume alterations are different from those in normotensive control animals. Radioiodated serum albumin and radiobromide will be used to determine plasma volume (PV) and extracellular fluid volume (ECF), and chronically implanted capsules will be used to determine interstitial fluid pressure (IFP) in intact conscious rats. Interstitial fluid volume (IFV) will be derived from ECF-PV. The acute changes in these variables following a large isotonic fluid load, will be used to obtain delta IFV/delta IFP (interstitial fluid compartment compliance). These variables will be studied in one and two kidney Goldblatt and spontaneously hypertensive rats during the progression of hypertension and after chronic alterations in body fluid volumes by water restriction and sodium loading. The results should lead to a better understanding of the relationship between body fluid volumes and arterial pressure.